At the bottom right of the page you linked, the game is included in a "GOGmix" named "GOG games using DOSbox", so I'd assume it's the DOSbox emulator being clev

At the bottom right of the page you linked, the game is included in a "GOGmix" named "GOG games using DOSbox", so I'd assume it's the DOSbox emulator being clever, rather than GOG's reverse-engineering skills. For example, DOSbox emulates the DOS API directly, rather than running a real copy of DOS and emulating the hardware. That means that DOSbox's DOS doesn't actually take up any space in the emulated machine, leaving vastly more of the magic 640KB available for applications to use without having to mess around with memory-managers.